The present invention relates to cellulose, liquid-crystal solutions having a base of cellulose, that is to say a high concentration of cellulose, as well as spun products of cellulose.
More particularly, the invention relates to liquid-crystal solutions having a base of cellulose when they are spinnable, that is to say solutions which can be spun in order to produce fibers of films having high mechanical properties.
It has been known for a long time that the production of liquid-crystal solutions is essential for the obtaining of fibers having high mechanical properties, as shown, in particular, by U.S. Pat. No. 3,767,756 relating to aramid fibers and U.S. Pat. No. 4,746,694 relating to fibers of aromatic polyesters.
Liquid-crystal solutions having a base of cellulose derivatives, as well as the fibers or films which can be obtained from these solutions have also been described in certain patents or patent applications.
U.S. Pat. Nos. 4,370,168 and 4,501,886 have, for instance, shown the existence of spinnable liquid-crystal solutions having a base of ethers or cellulose acetate dissolved in inorganic acids, which solutions make it possible to obtain fibers of high tensile strength and high modulus. The methods employed are, however, limited in their application by the following drawbacks:
the necessity of separately preparing the derivatives and then the solutions of these derivatives; PA1 after spinning the solutions of derivatives, the need to regenerate the fibers, when possible, in order to obtain cellulose fibers; PA1 no regeneration possible in the case of ethers; PA1 a very lengthy regeneration (several hours), preventing operation on-line with the spinning, in the case of the acetates. PA1 it contains more than 10% cellulose by weight; PA1 it is optically anisotropic at rest and at room temperature; PA1 the solvent agent contains more than 85% by weight of at least one phosphoric acid, this phosphoric acid or all of these phosphoric acids having the following average formula: EQU n(P.sub.2 O.sub.5), p(H.sub.2 O)!, PA1 with 0.33&lt;(n/p)&lt;1.0. PA1 0.5.ltoreq.(n/p).ltoreq.0.75. PA1 cellulose is placed in contact with a solvent agent which contains more than 85% by weight of at least one phosphoric acid, this phosphoric acid or all of these phosphoric acids having the average formula EQU n(P.sub.2 O.sub.5), p(H.sub.2 O)!, PA1 with 0.33&lt;(n/p)&lt;1.0; PA1 an amount of cellulose of more than 10% by weight of the mixture of cellulose plus solvent agent is used; PA1 this cellulose is dissolved directly by kneading with the solvent agent. PA1 the spinnable solution defined above is transformed into a liquid extrudate having the form of a fiber or film; PA1 the said liquid extrudate is coagulated in a coagulating medium; PA1 the fiber or film thus obtained is washed and then dried. PA1 the reinforcing assemblages such as, for instance, cables or plied yarns each comprising at least one spun product in accordance with the invention; PA1 the articles reinforced with the spun products in accordance with the invention, in particular the articles each reinforced by at least one fiber and/or film and/or a reinforcing assemblage in accordance with the invention, these articles being for instance articles of rubber or plastic, for instance plies, belts, tubes, and tire casings.
European Patent B 179,822 and the equivalent U.S. Pat. No. 4,839,113 describe the obtaining of liquid-crystal spinning solutions having a base of cellulose formate by reacting cellulose with formic acid and phosphoric acid. They also describe fibers of regenerated cellulose having high mechanical properties, obtained by regeneration of cellulose formate fibers. The processes used have numerous advantages. In particular, the use of the formate as derivative permits a derivation in situ (formylation and dissolution of the cellulose in the same solvent mixture) and a very rapid regeneration, on line with the spinning. These processes are nevertheless penalized by the recycling of a mixture of acids as well as of products of the regeneration.
Moreover, numerous attempts have been made to obtain liquid-crystal solutions of cellulose by dissolving the cellulose directly (no derivation and therefore suppression of the regeneration).
It has, for instance, been proposed to dissolve the cellulose in a mixture of N,N-dimethylacetamide (DMAC) and lithium chloride (LiCl), as described for example in U.S. Pat. No. 4,278,790. With such a system, it is not possible to obtain sufficiently concentrated solutions and their anisotropy is slight, so that it is not possible to obtain fibers which have high mechanical properties (see, for example J. Polym. Sci., Polym. Phys. Ed. 27:1477, 1989, and J. Appl. Polym. Sci. 51:583, 1994).
It has also been proposed to dissolve the cellulose directly in N-oxide tertiary amines, in particular the N-oxide of N-methylmorpholine, as described for example in U.S. Pat. No. 4,416,698. These solvents are poorly adapted to the use of highly concentrated solutions, a condition of concentration which moreover is necessary for the obtaining of a liquid-crystal phase since the temperature of use of these solutions is close to the degradation temperature of the solvent.
The Japanese patent application published under number 4-258 648 also describes solutions obtained by the direct dissolving of cellulose, some of these solutions being capable of presenting optical anisotropy under certain conditions. However, the solvent mixture proposed is formed of at least two acids, in particular sulfuric acid and phosphoric acid. The presence of two acids may result in serious complications upon recycling. Furthermore, the use of sulfuric acid entails the risk of leading to a sulfonation of the cellulose or to an extensive degradation of it, and therefore to more difficult spinning. There is also the risk of rapid corrosion of the equipment.